with every game that I start on Dolphin (Wii or GC), I'm experiencing slowed down and crackling audio when the framerate is below 60 FPS.
There were numerous posts over the years discussing crackling audio. However none of the solutions suggested worked for me.
I don't remember everything I've tried. I can say for sure, that I've tried absolutely everything in the audio settings.

I haven't had this problem when I played around with dolphin a couple of years ago.
You can check out my system in the profile, but I don't think it's an issue. At least specs wise.

You can use audio stretching to get rid of the crackling, but the audio will still be slowed down. If you want the audio to be at full speed, the emulation has to be at full speed as well. There is no way around that, unlike in old Dolphin versions with their hackier and buggier implementations of DSP HLE.

Mainly Super Mario Galaxy 1&2 and Mario Party 9.
ALso Wii Sports, Wii Play, Wario Ware.
I can't try them right now, because my Wiimote is at a friends house. He has no audio issues with Dolphin.
But in the introduction cinematics of the Mario Games the fps fluctuates below 60 FPS and so the audio speeds up and down.

I've just tried Smash brothers brawl, that is actually playable.
And I also just tried Zelda Occarina of Time again. It has just 10-15 FPS, so the audio is very slowed down.

Audio is tied to framerate in new builds of Dolphin. If you play a game in Dolphin and the game is not running full speed, then audio SHOULDN'T be full speed. In fact, that's why games like Wind Waker, Super Mario Galaxy, and many other games crashed in Dolphin.

So deal with the slowdown, or you can use a build before we fixed audio and deal with crashes or you can try to make Dolphin run faster.